Heat leakage into the LNG storage container vaporizes some of the liquid phase, increasing the container pressure. In the past, this pressure was relieved by consuming the LNG-containing gases which flashed off as auxiliary fuel to the steam boilers for steam driven LNG tankers. Alternatively, the flashed gas could be purged by venting or flaring when outside of port. Recent LNG tanker designs use diesel engine drives rather than steam driven engines. These new tankers have reliquefiers for recondensing LNG boiloff but and have no method for disposing of the reliquefier purge. Also, regulations prohibit disposal of hydrocarbon-containing streams by venting or flaring, especially while in port. With elimination of these options for controlling the boiloff, it has been proposed to recover the LNG by reliquefying the flashed gas and returning it to the LNG storage container.
During startup and turndown (reduced load) operation of a boiloff reliquefier for an LNG storage container, nitrogen (N.sub.2) impurities will flash preferentially from the LNG and concentrate within the vapor system. The primary source of nitrogen impurity is that which is contained originally in the natural gas, usually up to about 0.5 percent. Nitrogen, more volatile than LNG, flashes off preferentially and concentrates within the vapor system. For example, LNG containing 0.3 percent N.sub.2 will produce a vapor containing approximately 3% N.sub.2.
The reliquefaction of the flashed gas is hampered by the presence of the nitrogen impurity. Under the startup and turndown conditions, the boiloff reliquefier system concentrates nitrogen to the point at which the internal refrigerant system of the reliquefier can not provide sufficient refrigeration at a low enough temperature to reach the dew point of the flashed gas. At this point, reliquefaction ceases until the vapor phase N.sub.2 concentration is reduced.
A reliquefaction system is described by P. Wicker of Sulzer Brothers Limited, Switzerland in Reliquefaction of LNG Boiloff Gas, The Oil and Gas Journal, 53-55 (Jan. 18, 1971). This system utilizes a refrigerant buffer vessel whereby the refrigeration capacity can be reduced down to 30 percent of design capacity. At the end of page 54, the article outlines an attempted procedure for initial cooldown of the LNG storage tank, but states that such procedure failed. This failure was due to freezeup of the condenser with moisture and heavy hydrocarbons. Such experience demonstrates unanticipated problems when operating far from design conditions.